Over the last few decades, a wide variety of both wired and wireless telecommunication systems have been developed. Wireless telecommunication systems in particular have evolved through the so-called second generation (2G) systems into the third generation (3G) systems currently being deployed. Specifications for some 3G systems were developed by the 3rd Generation Partnership Project (3GPP); information regarding these may be found on the Internet at www.3gpp.org.
Continuing development of advanced wireless systems has produced techniques enabling even higher data transfer speeds. To this end, so-called High-Speed Downlink Packet Access (HSDPA) technology has recently been developed. HSDPA delivers packet data to a plurality of mobile terminals over a shared downlink channel at high peak data rates, and provides a smooth evolutionary path for 3G networks to support higher data transfer speeds.
HSDPA achieves increased data transfer speeds by defining a new downlink transport channel, the High-Speed Downlink Shared Channel (HS-DSCH), which operates in significantly different ways from other W-CDMA channels. In particular, the HS-DSCH downlink channel is shared between users, and relies on user-specific channel-dependent scheduling to make the best use of available radio resources. On a separate uplink control channel, each user device periodically transmits (e.g. as many as 500 times per second) an indication of the downlink signal quality. The Wideband-CDMA base station (Node B) analyzes the channel quality information received from all user devices to decide which users will be sent data on each 2-millisecond frame and, for each user, how much data should be sent in that frame. Using adaptive modulation and coding (AMC) techniques in addition to this frame-by-frame fast packet scheduling, more data can be sent to users which report high downlink signal quality. Thus, the limited radio resources are used more efficiently.
To support the newly defined HS-DSCH channel, three new physical channels are also introduced. First is the High-Speed Shared Control Channel (HS-SCCH), which is used to convey scheduling information to the user device. In essence, this scheduling information describes data that will be sent on the HS-DSCH two slots later. Second is the uplink High-Speed Dedicated Physical Control Channel (HS-DPCCH), which carries acknowledgement information transmitted by mobile terminals as well as current channel quality indicator (CQI) data for the user device. The CQI data is used by the Node B in its fast packet scheduling algorithms, i.e., in calculating how much data to send to the mobile terminal during the next transmission interval. Finally, a newly defined downlink physical channel is the High-Speed Physical Dedicated Shared Channel (HS-PDSCH), which is the physical channel carrying the user data of the HS-DSCH transport channel.
In addition to the fast packet scheduling and adaptive modulation and coding technologies discussed above, HSDPA further utilizes fast retransmissions for error control. In particular, HSDPA utilizes an error control method known as Hybrid Automatic Repeat Request, or HARQ. HARQ uses the concept of “incremental redundancy”, where retransmissions contain different coding of the user data relative to the original transmission. When a corrupted packet is received, the user device saves it, sends a “NACK” message to trigger a re-transmission of the packet, and combines the saved packet with subsequent retransmissions to formulate an error-free packet as quickly and efficiently as possible. Even if the retransmitted packet(s) is itself corrupted, the combining of information from two or more corrupted transmissions can often yield an error-free version of the originally transmitted packet.
In fact, HARQ is a variation of Automatic Repeat-reQuest (ARQ) error control, which is a well-known error control method for data transmission in which the receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. HARQ gives better performance than ordinary ARQ, particularly over wireless channels, at the cost of increased implementation complexity.
The simplest version of HARQ, Type I HARQ, simply combines Forward Error Correction (FEC) and ARQ by encoding the data block plus error-detection information—such as Cyclic Redundancy Check (CRC)—with an error-correction code (such as Reed-Solomon code or Turbo code) prior to transmission. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is poor and not all transmission errors can be corrected, the receiver will detect this situation using the error-detection code. In this case, the received coded data block is discarded and a retransmission is requested by the receiver, similar to ARQ.
In more advanced methods, incorrectly received coded data blocks are stored at the receiver rather than discarded, and when the retransmitted coded data block is received, the information from both coded data blocks are combined. When the transmitted and re-transmitted blocks are coded identically, so-called Chase combining may be used to benefit from time diversity. To further improve performance, incremental redundancy HARQ has also been proposed. In this scheme, retransmissions of a given block are coded differently from the original transmission, thus giving better performance after combining since the block is effectively coded across two or more transmissions. HSDPA in particular utilizes incremental redundancy HARQ, wherein the data block is first coded with a punctured Turbo code. During each re-transmission the coded block is punctured differently, so that different coded bits are sent each time.
ARQ schemes in general may be utilized in stop-and-wait mode (after transmitting a first packet, the next packet is not transmitted until the first packet is successfully decoded), or in selective repeat mode, in which the transmitter continues transmitting successive packets, selectively re-transmitting corrupted packets identified by the receiver by a sequence number. A stop-and-wait system is simpler to implement, but waiting for the receiver's acknowledgement reduces efficiency. Thus, in practice multiple stop-and-wait HARQ processes are often performed in parallel so that while one HARQ process is waiting for an acknowledgement one or more other processes can use the channel to send additional packets.
The first versions of HSDPA address up to 8 HARQ processes, numbered 0 through 7. This number is specified to ensure that continuous transmissions to one user may be supported. When a packet has been transmitted from the Node B, the mobile terminal will respond (on the HS-DPCCH) with an ACK (acknowledge) or NACK (not-ACK) indication, depending on whether the packet decoded correctly or not. Because of the inherent delay in processing and signaling, several simultaneous HARQ processes are required. The Node B transmitter thus is able to transmit several new packets before an ACK or NACK is received from a previous packet.
HSDPA as specified in 3GPP release 7 and forward is designed to achieve improved data rates of up to 28.8 Mbps. This is accomplished by introducing advanced multi-antenna techniques, i.e. Multiple-Input Multiple-Output (MIMO) technology. In particular, spatial multiplexing is employed to divide the data into two transmission streams, often called data substreams. These substreams are transmitted with multiple transmit antennas, using the same frequencies and the same channelization codes. Given uncorrelated propagation channels, receivers employing multiple receive antennas and using advanced detection techniques such as successive interference cancellation are able to distinguish between and decode the multiplexed data substreams.
With the addition of MIMO to HSDPA, the number of required HARQ processes increases, e.g. from 8 to 16 (0-15) processes. If the processes are independently numbered for each data substream and signaled to the receiving mobile terminals, the signaling load on the HS-SCCH will increase significantly. Instead of a 3-bit HARQ process number for identifying eight processes, a 4-bit HARQ process number is needed to distinguish between up to 16 processes. In a dual stream case, as currently under development for HSDPA systems, the signaling overhead would thus increase from three to eight bits (two streams at four bits/stream). Because signaling on HS-SCCH is relatively expensive, i.e., signaling bits are scarce, this increase in overhead is undesirable.